In recent years, non-destructive transillumination imaging using terahertz waves and terahertz time-domain spectroscopy for studying the characteristics of materials using terahertz wave pulses have been actively studied. PTL discloses a method that relates to image processing performed in terahertz imaging and that uses wavelet analysis for information compression and peak detection. Furthermore, NPL discloses performing a wavelet transform on a time waveform of a terahertz wave and setting a value of a wavelet expansion coefficient that is smaller than a threshold value to zero (threshold processing). Accordingly, a noise component included in the time waveform can be removed. By performing an inverse wavelet transform after removing the noise component, a time waveform with the noise component removed therefrom can be obtained. NPL discusses comparing S/N ratios after the removal of the noise component so as to determine which mother wavelet has the greatest amount of noise component removed therefrom among various mother wavelets. The noise component mainly expresses white noise occurring over the entire frequency range. Due to having a low correlation with a mother wavelet, the noise component occurs near zero with respect to each expansion coefficient.